<s>
In	O
semiconductor	O
physics	O
,	O
the	O
depletion	B-Algorithm
region	I-Algorithm
,	O
also	O
called	O
depletion	B-Algorithm
layer	I-Algorithm
,	O
depletion	B-Algorithm
zone	I-Algorithm
,	O
junction	O
region	O
,	O
space	B-Algorithm
charge	I-Algorithm
region	I-Algorithm
or	O
space	O
charge	O
layer	O
,	O
is	O
an	O
insulating	O
region	O
within	O
a	O
conductive	O
,	O
doped	B-Algorithm
semiconductor	O
material	O
where	O
the	O
mobile	O
charge	O
carriers	O
have	O
been	O
diffused	O
away	O
,	O
or	O
have	O
been	O
forced	O
away	O
by	O
an	O
electric	O
field	O
.	O
</s>
<s>
The	O
only	O
elements	O
left	O
in	O
the	O
depletion	B-Algorithm
region	I-Algorithm
are	O
ionized	O
donor	O
or	O
acceptor	O
impurities	O
.	O
</s>
<s>
This	O
region	O
of	O
uncovered	O
positive	O
and	O
negative	O
ions	O
is	O
called	O
the	O
depletion	B-Algorithm
region	I-Algorithm
due	O
to	O
the	O
depletion	O
of	O
carriers	O
in	O
this	O
region	O
.	O
</s>
<s>
The	O
depletion	B-Algorithm
region	I-Algorithm
is	O
so	O
named	O
because	O
it	O
is	O
formed	O
from	O
a	O
conducting	O
region	O
by	O
removal	O
of	O
all	O
free	O
charge	O
carriers	O
,	O
leaving	O
none	O
to	O
carry	O
a	O
current	O
.	O
</s>
<s>
Understanding	O
the	O
depletion	B-Algorithm
region	I-Algorithm
is	O
key	O
to	O
explaining	O
modern	O
semiconductor	O
electronics	O
:	O
diodes	O
,	O
bipolar	O
junction	O
transistors	O
,	O
field-effect	O
transistors	O
,	O
and	O
variable	O
capacitance	O
diodes	O
all	O
rely	O
on	O
depletion	B-Algorithm
region	I-Algorithm
phenomena	O
.	O
</s>
<s>
A	O
depletion	B-Algorithm
region	I-Algorithm
forms	O
instantaneously	O
across	O
a	O
p	O
–	O
n	O
junction	O
.	O
</s>
<s>
Therefore	O
,	O
when	O
N-doped	O
and	O
P-doped	O
semiconductors	O
are	O
placed	O
together	O
to	O
form	O
a	O
junction	O
,	O
free	O
electrons	O
in	O
the	O
N-side	O
conduction	O
band	O
migrate	O
(	O
diffuse	O
)	O
into	O
the	O
P-side	O
conduction	O
band	O
,	O
and	O
holes	O
in	O
the	O
P-side	O
valence	O
band	O
migrate	O
into	O
the	O
N-side	O
valence	O
band	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
majority	O
charge	O
carriers	O
(	O
free	O
electrons	O
for	O
the	O
N-type	O
semiconductor	O
,	O
and	O
holes	O
for	O
the	O
P-type	O
semiconductor	O
)	O
are	O
depleted	O
in	O
the	O
region	O
around	O
the	O
junction	O
interface	O
,	O
so	O
this	O
region	O
is	O
called	O
the	O
depletion	B-Algorithm
region	I-Algorithm
or	O
depletion	B-Algorithm
zone	I-Algorithm
.	O
</s>
<s>
Due	O
to	O
the	O
majority	O
charge	O
carrier	O
diffusion	O
described	O
above	O
,	O
the	O
depletion	B-Algorithm
region	I-Algorithm
is	O
charged	O
;	O
the	O
N-side	O
of	O
it	O
is	O
positively	O
charged	O
and	O
the	O
P-side	O
of	O
it	O
is	O
negatively	O
charged	O
.	O
</s>
<s>
When	O
the	O
electric	O
field	O
is	O
sufficiently	O
strong	O
to	O
cease	O
further	O
diffusion	O
of	O
holes	O
and	O
electrons	O
,	O
the	O
depletion	B-Algorithm
region	I-Algorithm
reaches	O
the	O
equilibrium	O
.	O
</s>
<s>
Integrating	O
the	O
electric	O
field	O
across	O
the	O
depletion	B-Algorithm
region	I-Algorithm
determines	O
what	O
is	O
called	O
the	O
built-in	O
voltage	O
(	O
also	O
called	O
the	O
junction	B-Algorithm
voltage	I-Algorithm
or	O
barrier	O
voltage	O
or	O
contact	O
potential	O
)	O
.	O
</s>
<s>
When	O
the	O
two	O
current	O
components	O
balance	O
,	O
as	O
in	O
the	O
p	O
–	O
n	O
junction	O
depletion	B-Algorithm
region	I-Algorithm
at	O
dynamic	O
equilibrium	O
,	O
the	O
current	O
is	O
zero	O
due	O
to	O
the	O
Einstein	O
relation	O
,	O
which	O
relates	O
D	O
to	O
σ	O
.	O
</s>
<s>
Forward	O
bias	O
(	O
applying	O
a	O
positive	O
voltage	O
to	O
the	O
P-side	O
with	O
respect	O
to	O
the	O
N-side	O
)	O
narrows	O
the	O
depletion	B-Algorithm
region	I-Algorithm
and	O
lowers	O
the	O
barrier	O
to	O
carrier	O
injection	O
(	O
shown	O
in	O
the	O
figure	O
to	O
the	O
right	O
)	O
.	O
</s>
<s>
When	O
bias	O
is	O
strong	O
enough	O
that	O
the	O
depletion	B-Algorithm
region	I-Algorithm
becomes	O
very	O
thin	O
,	O
the	O
diffusion	O
component	O
of	O
the	O
current	O
(	O
through	O
the	O
junction	O
interface	O
)	O
greatly	O
increases	O
and	O
the	O
drift	O
component	O
decreases	O
.	O
</s>
<s>
The	O
low	O
current	O
conducted	O
under	O
reverse	O
bias	O
and	O
the	O
large	O
current	O
under	O
forward	O
bias	O
is	O
an	O
example	O
of	O
rectification	B-Algorithm
.	O
</s>
<s>
Under	O
reverse	O
bias	O
(	O
applying	O
a	O
negative	O
voltage	O
to	O
the	O
P-side	O
with	O
respect	O
to	O
the	O
N-side	O
)	O
,	O
the	O
potential	O
drop	O
(	O
i.e.	O
,	O
voltage	O
)	O
across	O
the	O
depletion	B-Algorithm
region	I-Algorithm
increases	O
.	O
</s>
<s>
Thus	O
the	O
depletion	B-Algorithm
region	I-Algorithm
is	O
widened	O
and	O
its	O
field	O
becomes	O
stronger	O
,	O
which	O
increases	O
the	O
drift	O
component	O
of	O
current	O
(	O
through	O
the	O
junction	O
interface	O
)	O
and	O
decreases	O
the	O
diffusion	O
component	O
.	O
</s>
<s>
The	O
final	O
equation	O
would	O
then	O
be	O
arranged	O
so	O
that	O
the	O
function	O
of	O
depletion	B-Algorithm
layer	I-Algorithm
width	O
would	O
be	O
dependent	O
on	O
the	O
electric	O
potential	O
.	O
</s>
<s>
In	O
summary	O
,	O
and	O
are	O
the	O
negative	O
and	O
positive	O
depletion	B-Algorithm
layer	I-Algorithm
width	O
respectively	O
with	O
respect	O
to	O
the	O
center	O
,	O
and	O
are	O
the	O
amount	O
of	O
acceptor	O
and	O
donor	O
atoms	O
respectively	O
,	O
is	O
the	O
electron	O
charge	O
and	O
is	O
the	O
built-in	O
voltage	O
,	O
which	O
is	O
usually	O
the	O
independent	O
variable	O
.	O
</s>
<s>
Another	O
example	O
of	O
a	O
depletion	B-Algorithm
region	I-Algorithm
occurs	O
in	O
the	O
MOS	O
capacitor	O
.	O
</s>
<s>
Supposing	O
that	O
the	O
semiconductor	O
initially	O
is	O
charge	O
neutral	O
,	O
with	O
the	O
charge	O
due	O
to	O
holes	O
exactly	O
balanced	O
by	O
the	O
negative	O
charge	O
due	O
to	O
acceptor	O
doping	B-Algorithm
impurities	O
.	O
</s>
<s>
The	O
greater	O
the	O
positive	O
charge	O
placed	O
on	O
the	O
gate	O
,	O
the	O
more	O
positive	O
the	O
applied	O
gate	O
voltage	O
,	O
and	O
the	O
more	O
holes	O
that	O
leave	O
the	O
semiconductor	O
surface	O
,	O
enlarging	O
the	O
depletion	B-Algorithm
region	I-Algorithm
.	O
</s>
<s>
If	O
the	O
gate	O
material	O
is	O
polysilicon	O
of	O
opposite	O
type	O
to	O
the	O
bulk	O
semiconductor	O
,	O
then	O
a	O
spontaneous	O
depletion	B-Algorithm
region	I-Algorithm
forms	O
if	O
the	O
gate	O
is	O
electrically	O
shorted	O
to	O
the	O
substrate	O
,	O
in	O
much	O
the	O
same	O
manner	O
as	O
described	O
for	O
the	O
p	O
–	O
n	O
junction	O
above	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
both	O
and	O
can	O
be	O
viewed	O
as	O
doping	B-Algorithm
spatial	O
densities	O
.	O
</s>
<s>
The	O
depletion	B-Algorithm
region	I-Algorithm
is	O
not	O
symmetrically	O
split	O
between	O
the	O
n	O
and	O
p	O
regions	O
-	O
it	O
will	O
tend	O
towards	O
the	O
lightly	O
doped	B-Algorithm
side	O
.	O
</s>
<s>
A	O
more	O
complete	O
analysis	O
would	O
take	O
into	O
account	O
that	O
there	O
are	O
still	O
some	O
carriers	O
near	O
the	O
edges	O
of	O
the	O
depletion	B-Algorithm
region	I-Algorithm
.	O
</s>
<s>
In	O
the	O
MOSFET	B-Architecture
,	O
this	O
inversion	O
layer	O
is	O
referred	O
to	O
as	O
the	O
channel	O
.	O
</s>
<s>
Associated	O
with	O
the	O
depletion	B-Algorithm
layer	I-Algorithm
is	O
an	O
effect	O
known	O
as	O
band	O
bending	O
.	O
</s>
<s>
This	O
effect	O
occurs	O
because	O
the	O
electric	O
field	O
in	O
the	O
depletion	B-Algorithm
layer	I-Algorithm
varies	O
linearly	O
in	O
space	O
from	O
its	O
(	O
maximum	O
)	O
value	O
at	O
the	O
gate	O
to	O
zero	O
at	O
the	O
edge	O
of	O
the	O
depletion	O
width	O
:	O
</s>
